PIC18F2420/2520/4420/4520
instruction that executes from a location outside of that
block is not allowed to read and will result in reading ‘0’s.
Figures 23-6 through 23-8 illustrate table write and table
read protection.
23.5.1
PROGRAM MEMORY
CODE PROTECTION
The program memory may be read to, or written from,
any location using the table read and table write
instructions. The Device ID may be read with table
reads. The Configuration registers may be read and
written with the table read and table write instructions.
Note:
Code protection bits may only be written to
a ‘0’ from a ‘1’ state. It is not possible to
write a ‘1’ to a bit in the ‘0’ state. Code pro-
tection bits are only set to ‘1’ by a full chip
erase or block erase function. The full chip
erase and block erase functions can only
be initiated via ICSP or an external
programmer.
In normal execution mode, the CPn bits have no direct
effect. CPn bits inhibit external reads and writes. A block
of user memory may be protected from table writes if the
WRTn Configuration bit is ‘0’. The EBTRn bits control
table reads. For a block of user memory with the EBTRn
bit set to ‘0’, a table read instruction that executes from
within that block is allowed to read. A table read
FIGURE 23-6:
TABLE WRITE (WRTn) DISALLOWED
Register Values
Program Memory
Configuration Bit Settings
000000h
0007FFh
WRTB, EBTRB = 11
000800h
TBLPTR = 0008FFh
PC = 001FFEh
WRT0, EBTR0 = 01
TBLWT*
TBLWT*
001FFFh
002000h
WRT1, EBTR1 = 11
WRT2, EBTR2 = 11
WRT3, EBTR3 = 11
003FFFh
004000h
PC = 005FFEh
005FFFh
006000h
007FFFh
Results: All table writes disabled to Blockn whenever WRTn = 0.
DS39631E-page 264
© 2008 Microchip Technology Inc.
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